This invention relates to an exhaust gas purification apparatus in an internal combustion engine chiefly for a vehicle such as a motorized two-wheeled vehicle or the like.
As for an apparatus of this kind, one has been known wherein an exhaust passage of an engine is provided with a catalyzer for three components of the exhaust gas, an exhaust gas sensor such as an 0.sub.2 sensor or the like, and a secondary air introducing opening so that the exhaust gas to be fed to the catalyzer may have air added to it to have a predetermined air-fuel ratio. This is generally performed by introducing the secondary air through the introducing opening in accordance with an output signal of the sensor. It has been usual in the type of apparatus that the air-fuel ratio of the exhaust gas is controlled to be near a theoretical air-fuel ratio, but this is not always preferrable. Namely, the purification characteristics of the catalyzer are usually as shown in FIG. 10. In other words, a total purification rate of CO, HC, NO.sub.X becomes a maximum at the theoretical air-fuel ratio, but the maximum purification rate of each of the three components lies on the rich side or the lean side thereof. More in detail, the maximum purification rate is on the rich side in respect of NO.sub. X, while the same is on the lean side in respect of HC and CO. Therefore, it is not always preferrable that the exhaust gas is so controlled as to be near the theoretical air-fuel ratio. In other words, it is usual that the composition ratio of the three components in the exhaust gas is varied with changes in the operating condition of the engine. It often happens that, for instance, generation of CO, and HC is increased by engine operation under low load and low-speed range and generation of NO.sub.X is increased by engine operation under high load and high-speed range. Consequently, the exhaust gas output is not always in compliance with the varied condition in an apparatus where the exhaust gas is always controlled to be near the theoretical air-fuel ratio.